## Introduction to Blackbody Radiation

The path to quantum mechanics began in 1859 with the work of the German
physicist Gustav Kirchoff who studied the spectrum of light emitted by
the Sun. The spectrum is continuous with a gently varying intensity
along with many sharp, dark lines superimposed. One set of lines, the
so-called D-lines were interpreted as coming from the absorption of light from
the continuous spectrum emanating from
the interior of the Sun by sodium atoms at its surface. To understand
the nature of the continuous spectrum, Kirchoff began to consider the emission
and absorption of radiation by heated materials in general.

In the
mid-1880's the Austrian physicist, Ludwig Boltzmann, derived an expression for
the total energy density emitted by an ideal blackbody. The formula, now
known as the Stefan-Boltzmann law, stated that the total energy density is
proportional to the fourth power of the temperature. Another German
physicist, Wilhelm Wien, made great strides in finding the intensity
distribution function that would match the spectra of heated materials.
The distribution he suggested worked well for high frequencies and gave the
correct prediction for the wavelength at which the maximum intensity occurred,
but was later found to give wrong predictions in the long wavelength, infrared
region.

The revolutionary work, published in 1900, of the German physicist, Max Planck, succeeded in
giving an intensity distribution that was in good agreement with all
experimental data. In order to derive his result, however, Planck had to
admit that classical physics was wrong. He had to make a drastic, quite
unjustified assumption: that the oscillators could only emit and absorb energy
in discrete units called quanta.

Refer to the following sections in __ Modern Physics__ by Bernstein, Fishbane and
Gasiorowicz:

Part 2: Historical Introduction, page 101.

Section 4-4: Derivation of blackbody formula, page 116.

Section 12-8: Blackbody radiation and the Bose-Einstein distribution of
photons, page 353.

Section 18-4: Cosmic background radiation, page 555.

**© 2000 by Prentice-Hall, Inc. A Pearson Company**